1. Centralized Base Station Technique Based on Remote Radio Frequency Units
In a mobile communication system, as shown in FIG. 1a, a wireless access network is typically composed of base stations (BTS) and a base station controller (BSC) or wireless networks controller (RNC) for controlling the base stations. As shown in FIG. 1b, a base station is mainly composed by a baseband processing subsystem, a radio frequency (RF) subsystem, antennas and etc., and performs transmission, reception and processing of wireless signals. The base station may cover different cells through a plurality of antennas.
In the mobile communication system, there are wireless network coverage problems that are more difficult to solve with conventional BTS technology, such as indoor coverage of high-rise buildings, coverage hole, or the coverage of shadow zone. The technique based on remote radio frequency units is a more effective solution being proposed to solve the above problems. In the base station system based on remote radio frequency units, main radio frequency units and antennas are installed in regions where it is required to provide a coverage, and are connected to other units in the base station through wideband transmission lines.
The technique may be further developed as the centralized base station technique based on remote radio frequency units. as compared to the conventional base station, the centralized base station based on remote radio frequency units has many advantages: allowing to replace one macro cell based on the conventional base station with a plurality of micro cells, thereby best accommodating different wireless environments and increasing wireless performances such as capacity, coverage and etc. of the system; the centralized structure makes it possible to perform soft handoff in the conventional base station by softer handoff, thereby obtaining an additional processing gain; and the centralized structure also makes it possible to use costly baseband signal processing resources as a resource pool shared by a plurality of cells, thereby obtaining benefits of statistical multiplexing and reduced system cost. United States patents, U.S. Pat. No. 5,657,374, “Cellular system with centralized base stations and distributed antenna units”, U.S. Pat. No. 6,324,391, “Cellular communication with centralized control and signal processing”, and etc. describes relevant implementation details about this technique.
As shown in FIG. 2, centralized base station system 10 based on remote radio frequency units is mainly composed of centrally installed central channel processing main unit (MU) 11 and remote radio frequency units (RRUs) 13 which are connected through wideband transmission links or network 12. BSC/RNC interface unit 14 in the central channel processing main unit 11 is in charge of performing user plane and signaling plane processing of BTS and BSC/RNC interface. The central channel processing main unit 11 further comprises channel processing resource pool 15, signal routing distribution unit 16 and etc., wherein the channel processing resource pool 15 is formed by stacking a plurality of channel processing units 1-N, and performs tasks such as baseband signal processing, and the signal distribution unit 16 dynamically allocates channel processing resources according to traffics of different cells to realize effective sharing of the processing resources among multiple cells. Besides the implementation inside the MU as shown in FIG. 2, the signal routing distribution unit 16 may also be implemented as a separate device outside the MU. The remote antenna element is mainly constituted by functional units such as the transmission channel's radio frequency power amplifier, the reception channel's low noise amplifier, duplexer, antennas and etc. The link between the central channel processing subsystem and the remote antenna element may adopt transmission medium such as optical fiber, coaxial cable, microwave and etc.; the signal transmission may be done by way of digital signals after sampling, or analog signals after modulating; And the signals may be baseband signals, intermediate frequency signals or radio-frequency signals.
In the above two kinds of base station system based on remote radio frequency units, it is important to achieve wireless signal transmission between the RRU and the MU. It is typical to use analog Intermediate Frequency transmission solution or analog radio frequency signal transmission solution. Although it is easier to adopt analog signal transmission, the analog line will certainty introduce interference components such as noise, and the signal modulation during the transmission will also introduce nonlinear distortion. In addition, the analog transmission reduces transmission line utilization, and is inconvenient for application of high-capacity multiplexing technique. Therefore, it is difficult to form a large scale network with the analog transmission solution.
To this end, China patent applications, CN1464666, “Soft base station system based on fiber optic stretch and synchronous method thereof”, CN1471331, “Base station system for mobile communication” and etc. propose a solution of using digital signal transmission. To reduce the demand on transmission bandwidth as far as possible, it is usual to adopt the digital baseband signal transmission solution. CN1464666 only discloses a simple method of transmitting digital I/Q (in phase/quadrature) baseband signal between the RRU and the main base station by using optical fiber, that is, a digital I/Q baseband signal undergoes parallel to serial conversion to become a serial data stream at the transmitting end, is transmitted to the receiving end via an optical transmitter, and at the receiving end, undergoes serial to parallel conversion after reception to be recovered into the digital I/Q baseband signal; CN1471331 proposes a technique using Ethernet on physical layer, not using Ethernet MAC (media access control) frame, but using a transmission technique having particularly defined consecutive bit stream format. Presently, a cooperation organization named CPRI (Common Public Radio Interface) is also working on standardization of digital baseband transmission between the RRU and the main base station, and its technical specification can be downloaded from website http://www.cpri.info/spec.html. The specification adopts a technique similar to CN1471331, i.e., the physical interface adopts giga or 10GB Ethernet standard, and upper layers adopt self-defined consecutive bit stream format, but CPRI only supports point-to-point link. Since the above existing techniques adopt dedicated protocol specifications in transport layer technique without using a sophisticated transmission technique, many potential technique problems needs to be verified in actual systems, technical development and product development cycle is longer, network construction cost is higher. This is disadvantageous for the application of large scale network, and is also disadvantageous for protecting existing investment of operators.
2. Load-Sharing Technique in the Wireless Base Station System
In a patent application entitled “load-sharing method and system in a wireless base station” filed in China patent office on Mar. 4, 2004, the same inventors propose a method of channel processing load-sharing between wireless base stations, where the base stations are connected through wideband links to form a distributed wireless base station system, and when the traffic amount of cells served by a base station is too heavy or a part or all of its channel processing units fail, it is possible to route or switch, through the wideband links between the base stations, wireless signals corresponding to the part or all of cells served by the base station to one or more other remote end base stations connected therewith, and to share the part or all of processing load of the base station by the other remote end base stations, thereby reducing call loss of the entire base station system due to inadequate resource, and facilitating to increase resource utilization and reliability of the entire base station system.
Since the wideband links for transmitting wireless signals between the base stations have properties similar to that between the RRU and the MU in the above base station system based on remote radio frequency units, the wireless signal transmission technique between the RRU and the MU in the base station system based on remote radio frequency units is also applicable to the wireless signal transmission between the wireless base stations having load-sharing structure.
3. Origination of the Invention
In sum, there is a need for transmission of wireless signals between MU and RRU and between BTSs in the wireless base station system. However, there are still some problems existing in the prior art, and on the other hand, techniques for transmitting high-capacity IP packets on wide area network, such as IP over SDH (synchronous digit sequence), IP over DWDM (dense wave division multiplex) and etc. have been grown up and used widely. As the development of wideband technique such as IEEE 802.3ab giga Ethernet (GE), IEEE 802.3ae 10 giga Ethernet (10GE) and etc., it is possible to build a packet exchange network of high speed, wideband and cheaper than conventional telecommunication transmission network within range of local area network (LAN). Further, because of the emergence of technique for directly carrying Ethernet MAC frames on wideband transmission network such as SDH, DWDM and CWDM (coarse wave division multiplex), and the support of optional wide area network (WAN) interface sublayer in 10GE to VC-4-64c container of direct transmission SDH, the wideband packet exchange network based on Ethernet is extended from local area network application to wide area network.
Therefore, if digital wireless signals between the RRU and the MU in the base station system based on remote radio frequency units can be transmitted by using a wideband packet exchange network of relatively low cost and compatible with the existing techniques, it will be beneficial for implementing a lower cost, fast, flexible large scale networking application. To this end, the present invention proposes a method of packet transmission of wireless signals (Radio over Packet) in the wireless base station system.